Plastic spectacle lenses mainly used as spectacle lenses at present are produced by two methods. According to one method, a finished lens is directly formed by cast molding, wherein both surfaces of the lens are formed into final optical surfaces of a spectacle lens by transferring surface shapes of molds thereto. According to the other method, a thick semi-finished lens is previously formed by cast molding, wherein one surface of the lens is finished into a final optical surface by transferring a surface shape of one of molds thereto, and then the other surface of the lens is formed into a specific lens surface shape by cutting and polishing.
Spectacle lenses are classified into two major types: single-focal lenses and multi-focal lenses. The most popular kind of multi-focal lenses is a progressive power lens having a progressive surface composed of a distance portion, a near portion, and a progressive portion in which a focal length is continuously changed between the distance portion and the near portion. A single-focal lens basically has factors of a spherical power, an astigmatic (cylindrical) power, and a lens thickness, and a multi-focal lens basically has factors of a spherical power, an astigmatic (cylindrical) power, an astigmatic axis, an addition (ADD) power, and a lens thickness. The number of combinations of these factors becomes very large, particularly, significantly large for multi-focal lenses. Accordingly, the method of directly forming a finished lens by cast molding has been limited to production of frequently ordered spectacle lenses having popular combinations of the above factors, and many plastic lenses have been produced by forming finished lenses from semi-finished lenses by cutting.
In the case of adopting the method of forming finished lenses from semi-finished lenses by cutting, it is required to previously prepare the semi-finished lenses by molding. A semi-finished lens is produced by cast molding using two molds. In the cast molding, the convex side of the lens is formed into a spherical surface (for a single-focal lens) or a progressive surface (for a progressive power lens) by transferring a shape of one of the two molds thereto, and the concave side of the lens is formed into a shape, which is thicker than a finish size so as to be cut into any of finish shapes requested by prescriptions in a certain range, by transferring a shape of the other mold thereto. The concave surface of the semi-finished lens is then formed into a shape, which is a spherical surface shape (for a spherical lens) or a desired toric surface shape (for an astigmatic lens) and which has a specific thickness, by rough-cutting using a so-called curve generator or a generator capable of forming a pseudo toric surface. The concave surface of the semi-finished lens, which has been subjected to rough-cutting, is subjected to a copy lapping work similar to a so-called lapping work, to be accurately finished. The copy lapping step involves mounting a lens held on a specialized jig onto a working dish made from aluminum or the like on which a polishing pad is previously stuck, strongly pressing the working dish to the lens while pouring a lapping solution to a surface of the lens, and lapping the working dish relative to the lens, thereby transferring the surface shape of the working dish to the lens surface. A final lens surface shape with less roughness is obtained in the copy lapping step. The roughness of the lens surface is then smoothened by using an apparatus similar to that used in the copy lapping step, to obtain a final optical surface of the lens.
In recent years, however, there has been proposed a so-called inner surface progressive power lens in which a progressive surface or a curved surface obtained by synthesizing a progressive surface with a toric surface is provided on a concave surface on the eyeball side. Such an inner surface progressive power lens is advantageous in reducing fluctuation and distortion as faults of a progressive power lens on which a progressive surface is formed on the outer surface side, thereby significantly improving optical performances of the lens.
However, the prior art curve generator, which has been used for creating a concave surface on the eyeball side, can create only either a spherical surface or a toric surface because of its machining structure, and cannot create a progressive surface or a complex curved surface obtained by synthesizing a progressive surface with a toric surface on the concave surface. Further, the copy lapping work, which has a principle of lapping a working dish with a lens surface to transfer the shape of the working dish on the lens surface, cannot create a complex curved surface such as a progressive surface.
Accordingly, it has been required to develop a new method of producing a spectacle lens, which is capable of producing lenses having complex curved surfaces, such as an inner surface progressive power lens, with high productivity.
It has been also required to develop a polishing tool capable of mirror-polishing a complex curved surface created by the above-described method of producing a spectacle lens.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a method of producing a spectacle lens, which is capable of producing all kinds of spectacle lenses including an inner surface progressive power lenses with high productivity.
Another object of the present invention is to provide a polishing tool capable of mirror-polishing a complex curved surface.